1. Field of the Invention
This invention relates to a semiconductor wafer surface treatment method, and more particularly to a method for forming mechanical damages for contamination material-gettering on one surface of the wafer and for simultaneously cleaning the surface thereof.
2. Description of the Related Art
As the semiconductor element becomes increasingly available in more compact sizes, it becomes more important to reduce the influence of various types of contamination in the element manufacturing process. For example, contamination due to heavy metal such as Cu and Fe reduces the life time of minority carriers, changing the transistor characteristic and increasing the leak current. Further, contamination due to Na produces mobile ions in the oxide film, making the operation of the element unstable.
A so-called gettering method is known in the art for preventing the semiconductor wafer from becoming contaminated during manufacturing process. The gettering method includes a phosphor method and an HCl oxidation method, both of which are effected in the manufacturing process. Further, another gettering method is also known which provides gettering capability to the silicon wafer itself which is used as starting material. Such a gettering method includes an intrinsic gettering method utilizing oxygen precipitation caused inside the wafer and a back side damage (BSD) processing of lapping the back-side surface (opposite to the main surface in which semiconductor elements are formed) of the wafer and forming mechanical damages or distortion on the back-side surface of the wafer. In the heat treatment process, dislocation and stacking fault caused by the damage formed in the lapping and BSD processes may occur in the back-side surface of the wafer, and as a result, an area in which impurities of high diffusion speed are captured is provided, thus attaining the gettering effect.
The method for forming damage on the back-side surface of the wafer by lapping process has a disadvantage that the wafer will be warped after the lapping process. A sand blasting method is known as the BSD method and is used to form mechanical damage by blasting solution having fine particles of alumina dispersed therein to the back-side surface of the silicon wafer. The method is effective to mechanically form damages, but has a disadvantage that high cleanliness required for the silicon wafer used for the semiconductor element manufacturing process cannot be attained. That is, the solution having fine particles of alumina dispersed therein contains metal impurities such as Fe, Cr, Ni and Cu although they are small in amount. It is frequently determined by surface analysis by use of an ion micro-analyzer that the metal impurities are fixedly attached as surface contamination material to the wafer after the processing. Further, the wafer is finished with rough surface to provide surface irregularities. Since the solution containing alumina is blasted onto the irregular surface, fine particles of alumina run into and held in the back-side surface of the wafer. As a result, process defects will be caused in the semiconductor element manufacturing process in the same manner as in the case of surface contamination due to metal impurities.
Further, it is common practice to process the semiconductor wafers in a clean room from the view point of contamination control. However, since the sand blast method is effected by blasting solution containing fine particles of alumina at a high pressure, it becomes difficult to maintain the operation atmosphere of high cleanliness.
Thus, in the conventional semiconductor wafer surface treatment method, it is substantially impossible to maintain the high cleanliness when mechanical damages are formed on the back-side surface of the wafer.